Each year approximately 500,000 human joints require replacement as a result of debilitating disease or damage from an accident. Hip and knee joints represent a majority of these. To meet this need, a variety of total joint orthopaedic implants have been designed and are presently being marketed by various manufacturers. Examples of these are shown, for example, in U.S. Pat. Nos. 5,021,063 to Tager; 5,030,238 to Nieder et al.; 5,108,452 to Fallin; and 5,180,394 to Davidson.
Total joint orthopaedic implants are composed of two articulating components. The first of these components includes a concave surface made from a polymer or a ceramic (usually the former). The second of these components includes a convex counterface, typically formed of either metal or ceramic that is designed to slide or roll over the polymer or ceramic surface.
Ultrahigh molecular weight polyethylene remains the dominant polymer for utilization in the construction of concave bearing surfaces in total joint orthopaedic implants at the present time. To prevent undue wear to this bearing surface, the hard metal or ceramic counterface of the other component of the total joint orthopaedic implant should have an exceptionally smooth surface as roughness is directly related to the wear rate. The average surface roughness of, for example, a hip joint is about 0.025 .mu.m. It should be appreciated, however, even such a smooth hard, metal or ceramic surface generates wear particulates from the softer polymer surface. These wear particulates, unfortunately have been shown to be associated with adverse tissue reactions over time.
Specifically, the wear particulates stimulate cellular activity in the form of an immune system response. More specifically, inflammation and foreign body reactions result including macrophage and foreign body giant cell activity. This process leads to the production of bone resorbing cytokines (e.g. IL-1, IL-6, P6E-2, etc.) which lead to destruction of the bone tissue holding the implant securely in place. These conditions likely contribute to the loosening of prosthetic components eventually resulting in pain and failure of the total joint orthopaedic implant.
The shape, size and volume of the wear particulates produced may determine the type and extent of the adverse tissue and cellular reactions that are stimulated. It is hypothesized that very small wear particulates are transported away from the implant site and eliminated by the lymphatic system. More specifically, these very small particulates are phagocytized and transported through the lymphatic system by macrophages. The volume or rate of the particulates that the macrophages can phagocytize and transport, however, may be less than the volume or rate of particle generation. Furthermore, this volume or rate may depend upon the material composition of these small particulates, as well as their shape or size. Accordingly, the lymphatic system may become overloaded as a result of increased wear rates with the resulting particulate generation leading to an accumulation of wear particulates in the tissues around the total joint orthopaedic implant. This may produce a chronic inflammatory response that eventually leads to bone resorption.
Although larger wear particulates generally remain in the tissue surrounding the implant, they also can produce a chronic foreign body response which may eventually give rise to bone resorption. Accordingly, it should be appreciated that wear particulates have been repeatedly associated with aseptically loosened implants, a condition that eventually requires total joint orthopaedic implant replacement surgery. A need is therefore identified for a procedure to reduce the number, or modulate the shape and size of wear particulates generated in a human from a total joint orthopaedic implant. By successfully addressing this need, it is possible to increase the service life of the total joint orthopaedic implant and thereby reduce the need for difficult, painful and costly revision surgery.
Recognizing the problem resulting from wear particulate generation, a number of solutions have been proposed. As set out in the article entitled "Clinical Reviews: Particulate Debris And Failure of Total Hip Replacements" by Murali Jasty in the Journal of Applied Biomaterials, Volume 4, pp 273-276 (1993), these approaches include: the use of polyethylene components with more than 8 mm wall thickness and the use of head sizes smaller than 32 mm; the use of non-titanium alloys; the use of circumferential porous coatings on the femoral components; the use of prosthetic designs and surgical techniques to obtain rigid initial fixation at surgery, the minimizing of the modularity of the components and the surface hardening of the metal surface with techniques such as nitrating or ion implantation.
While each of these approaches is successful to some degree, none is directed to removing laps, folds and other surface irregularities (which form wear debris and would otherwise be produced in the body after implantation) prior to the implanting of the total joint orthopaedic implant into a patient. Further, while some clinical studies of the generation of wear particulates in a total joint implant have been completed and still others are ongoing, there has been absolutely no suggestion in the art that any method could be utilized to pre-wear a pair of matched implant joint bearing surfaces prior to implantation so as to significantly reduce the amount, rate of generation, or change the shape or size of wear particulates generated following implantation and thereby increase the service life of the total joint orthopaedic implant.
In addition, none of the known approaches are directed to increasing the conformity between the bearing surfaces of the articulating components which comprise the total joint orthopaedic implant. Particularly, there is no suggestion in the art that a method of pre-loading a pair of matched implant joint bearing surfaces, either above or with a pre-wearing process could be utilized to increase the conformity and to reduce the amount or change the shape or size of wear particulates generated following implantation and thereby increase the service life of the total joint orthopaedic implant.